* à (pronounced "ah") when it is used to indicate a destination, direction, or purpose. For example: "Je vais à la plage" (I'm going to the beach).
* de (pronounced "duh") when it is used to indicate origin, source, or possession. For example: "C'est un livre de mon frère" (It's a book from my brother).
It's important to consider the context of the sentence to determine which spelling to use.